IDT8T49N008ANLGI REVISION A FEBRUARY 13, 2014 29 ©2014 Integrated Device Technology, Inc.
IDT8T49N008I Data Sheet PROGRAMMABLE FEMTOCLOCK
®
NG LVPECL/LVDS CLOCK GENERATOR WITH 8-OUTPUTS
PCI Express Application Note
PCI Express jitter analysis methodology models the system
response to reference clock jitter. The block diagram below shows the
most frequently used Common Clock Architecture in which a copy of
the reference clock is provided to both ends of the PCI Express Link.
In the jitter analysis, the transmit (Tx) and receive (Rx) serdes PLLs
are modeled as well as the phase interpolator in the receiver. These
transfer functions are called H1, H2, and H3 respectively. The overall
system transfer function at the receiver is:
The jitter spectrum seen by the receiver is the result of applying this
system transfer function to the clock spectrum X(s) and is:
In order to generate time domain jitter numbers, an inverse Fourier
Transform is performed on X(s)*H3(s) * [H1(s) - H2(s)].
PCI Express Common Clock Architecture
For PCI Express Gen 1, one transfer function is defined and the
evaluation is performed over the entire spectrum: DC to Nyquist (e.g
for a 100MHz reference clock: 0Hz – 50MHz) and the jitter result is
reported in peak-peak.
PCIe Gen 1 Magnitude of Transfer Function
For PCI Express Gen 2, two transfer functions are defined with 2
evaluation ranges and the final jitter number is reported in RMS. The
two evaluation ranges for PCI Express Gen 2 are 10kHz – 1.5MHz
(Low Band) and 1.5MHz – Nyquist (High Band). The plots show the
individual transfer functions as well as the overall transfer function Ht.
PCIe Gen 2A Magnitude of Transfer Function
PCIe Gen 2B Magnitude of Transfer Function
For PCI Express Gen 3, one transfer function is defined and the
evaluation is performed over the entire spectrum. The transfer
function parameters are different from Gen 1 and the jitter result is
reported in RMS.
PCIe Gen 3 Magnitude of Transfer Function
For a more thorough overview of PCI Express jitter analysis
methodology, please refer to IDT Application Note PCI Express
Reference Clock Requirements.
Ht s H3 s H1 s H2 s–=
Ys Xs H3 s H1 s H2 s–=
